Production of silicon wafers for the manufacture of semiconductor chips requires that the wafer be ground to an exact diameter and that the periphery or outer edge of the wafer be defect-free down to a microscopic level. As part of the production process, a wafer is typically provided with a chamfer on its outer edge to reduce stress on the edge, which in turn helps prevent chipping and cracking of the wafer. The wafer also typically receives a chemical etch, followed by high temperature annealing in an inert gas atmosphere, such as nitrogen. However, the chemical etch step typically leaves a somewhat roughened surface on the chamfered edge of the wafer. Recently, particularly in the production of large diameter wafers, the wafers have been edge polished with standard polishing slurry solutions to smooth out the roughened chamfered edge resulting from the etching, and further to reduce stress caused by edge grinding to the precise diameters required. Edge polishing results in a mirror-like surface that resists adhesion of contaminating particulates.
However, such edge polishing is by means of an aqueous alkaline colloidal silica-containing slurry which provides the abrasive for the polishing process. This alkaline slurry occasionally splashes on to the surface of the wafer, causing a modest degree of unwanted etching both on the wafer surface and on the edge of the wafer, and becomes especially noticeable over time as evaporation of water in the slurry increases the concentration of caustic in the slurry. When the resulting unwanted etching is visible by halogen light inspection, the wafer is rejected as unacceptable product for so-called "edge stain," a micro-defect on the surface of the wafer that appears as microscopic pitting.
Chemical cleaning of silicon wafers by an aqueous ammoniacal solution, commonly referred to as an "SC-1" cleaning is known to be effective in preventing edge stain. However, SC-1 solutions are expensive and such a process step requires subsequent chemical and waste water treatment.
Although it is well known that a silicon oxide layer may be formed on wafers by exposing them to oxygen at elevated temperatures (McGuire, Semiconductor Materials and Process Technology Handbook, pp. 46-77 (1988)), it is also reported that alkaline impurities such as sodium and potassium salts can diffuse rapidly through the oxide layers so formed, even at low temperature, and accordingly it is recommended that more dense dielectric films such as silicon nitride be used in combination with the oxide to passivate the wafers against such impurities. Ibid., p. 55. Since an edge polishing slurry is alkaline, containing sodium and/or potassium salts, the prior art suggests that formation of a silicon oxide layer on a wafer would be of no value in preventing edge stain.
The present invention eliminates the need for a separate high temperature annealing step and provides a simple and efficient method of preventing edge stain.